This Newfoundland Breeding Colony maintained by Flow Laboratories, Inc., NIH 263-76-C-0551 CC has been developed from 15 dogs affected with varying degrees of discrete subaortic stenosis. More that 50% of dogs produced have developed varying degrees of severity of this defect with terminal cases resulting in sudden death. Cardiac catherizations have demonstrated peak systolic pressure gradients between left ventricle and aorta from 35 to 150 mm Hg in affected animals. Necropsy findings demonstrated severe left ventricular hypertrophy, discrete subaortic stenosis, severe abnormality of intramural coronary arteries in the ventricular septum, minor degrees of myocardial fiber disorientation and in several animals infective endocarditis. Maintenance of animal breeding and rearing of puppies is complicated by the necessity of artificial insemination of females and the artifical rearing of newborn pups. Sufficient numbers of animals are being bred to allow availability of this animal model for IR research programs in the near future.